The expression “intaglio printing” as used herein refers to direct plate printing using engraved printing mediums (e.g. engraved plates or cylinders) as commonly applied for the production of banknotes and security documents. The expression “steel/copper engraving printing” is also sometimes used to refer to this particular printing process. Banknotes, for example, are typically provided with intaglio-printed patterns (such as portraits or other pictorial representations, latent images, guilloche patterns, denominations, bank designation, etc.) which are readily recognizable by touch due to the embossing and tactile effect inherent to intaglio printing. Intaglio printing presses for carrying out intaglio printing are for instance known from European Patent Applications Nos. EP 0 091 709 A1, EP 0 406 157 A1, EP 0 563 007 A1, EP 0 873 866 A1, EP 1 602 482 A1 and International Application No. WO 03/047862 A1, all in the name of the present Applicant and incorporated herein by reference.
Intaglio printing requires application of an excess of ink onto the engraved printing mediums and wiping of the thus inked printing mediums prior to printing in order to remove the excess of ink outside of the engravings. In order to carry out this wiping operation, intaglio printing presses are commonly equipped with so-called wiping systems that typically comprise a wiping roller assembly as disclosed in the above-identified patent applications. One example of such a wiping system comprising a wiping roller assembly is disclosed in International Application No. WO 2007/116353 A1 in the name of the present Applicant which is also incorporated herein by reference.
There is no particular need to presently discuss the structure of wiping systems in more detail as the particular structure of such wiping systems does not directly impact on the subject-matter of the instant invention. For the purpose of the present invention, it suffices to understand that wiping systems for intaglio printing presses typically make use of so-called wiping solution (as for instance discussed in International Application No. WO 2007/116353 A1) to assist the wiping operation and the removal of ink residues from the wiping system and to ensure proper printing quality.
Wiping systems that do not make use of wiping solution are also known in the art but are now rarely used in contemporary intaglio printing presses.
Installations and processes for recycling the wiping solution of intaglio printing presses have been disclosed. The general purpose thereof is to limit the consumption of fresh wiping solution by processing waste wiping solution coming from the intaglio printing press in such a way as to remove ink constituents and like residues contained in the waste wiping solution to produce clean solution that can be recycled to produce fresh wiping solution.
European Patent Application No. EP 0 826 502 A1 in the name of the present Applicant, the content of which is incorporated herein by reference, for instance discloses an installation and process for recycling wiping solution of an intaglio printing press, which process involves:                i. ultra-filtration of the waste solution coming from one or more intaglio printing presses in order to produce clear solution and a concentrated residual solution;        ii. recycling of the clear solution derived from the ultra-filtration as fresh wiping solution, which fresh wiping solution typically contains softened water, sulfonated castor oil (S.C.O.) and sodium hydroxide (NaOH), also referred to as “caustic soda”;        iii. flocculation of the concentrated residual solution derived from the ultra-filtration using a flocculating agent, such as a mixture of calcium chloride (CaCl2) and ferric chloride (FeCl3), to produce a flocculated solution;        iv. filtration of the flocculated solution to produce solid waste and a filtered solution;        v. neutralisation of the filtered solution with a neutralising agent, such as hydrochloric acid (HCl) or sulphuric acid (H2SO4), to produce a neutralised solution;        vi. evaporation of the neutralised solution to produce a concentrated salty solution and distilled water;        vii. use of the distilled water for producing the fresh wiping solution; and        viii. filtration of the salty solution producing solid salts and a residual liquid, which residual liquid may be used for realising the flocculation at step iii. hereabove.        
European Patent Application No. EP 0 927 632 A1 in the name of the present Applicant, the content of which is incorporated herein by reference, discloses an improved installation and process for recycling wiping solution of an intaglio printing press, which process involves:                i. flocculation of the waste solution coming from one or more intaglio printing presses using a flocculating agent with particular physicochemical properties in order to produce flocculated solution containing flocculated ink constituents;        ii. separation by decantation of the flocculated solution to produce clean solution and a decanted solution with sedimented ink constituents;        iii. filtration of the decanted solution to produce filtered clean solution and solid waste;        iv. optionally, additional filtering (by ultra-filtration or mechanical filtration) of the clean solution derived from step ii. and of the filtered clean solution derived from step iii.; and        v. recycling of the clean solution derived from step ii. and of the filtered clean solution derived from step iii. as fresh wiping solution, which fresh wiping solution contains softened water, sulfonated castor oil (S.C.O.) and caustic soda (NaOH), including preferably an adjustment of the concentration of sulfonated castor oil (S.C.O.) and caustic soda (NaOH) additives.        
In both of the above examples, the fresh wiping solution typically contains approximately 98.5% of softened water, approximately 1% of caustic soda (NaOH) and approximately 0.5% of sulfonated castor oil (S.C.O.).
Further solutions for recycling wiping solution of intaglio printing presses are disclosed in European Patent Applications Nos. EP 0 789 000 A1, EP 1 832 333 A1, Japanese Patent Application No. JP 01-067296 A and Chinese Patent Application No. CN 1107075 A. All these other solutions are not based on any flocculation process and specifically involve an ultra-filtration step whereby the waste solution is filtered using inorganic (e.g. ceramic) or organic ultra-filtration membranes (or “UF membranes”) to produce a clean filtrate than can be recycled to produce fresh wiping solution. A problem with this approach resides mainly in the fact that the pores of the UF membranes are prone to clogging, which requires frequent cleaning and, ultimately, replacement of the UF membranes. This negatively impacts on operative and maintenance costs. In that respect, use of a flocculating agent to induce flocculation of ink constituents contained in the waste wiping solution is not practical in conjunction with an ultra-filtration process as flocculation exacerbates even further clogging of the UF membranes. This is one of the reasons why, according to European Patent Application No. EP 0 826 502 A1, flocculation is performed only after ultra-filtration.
Ultra-filtration is also contemplated in European Patent Application No. EP 0 927 632 A1 to perform additional filtration of the clean solution, but this approach is also prone to clogging of the UF membranes as there always remains some ink residues in the processed solution that is subjected to ultra-filtration. According to European Patent Application No. EP 0 927 632 A1, this problem can be overcome by performing a mechanical filtration in lieu of the ultra-filtration.
There is therefore a tendency to move away from ultra-filtration when it comes to recycling wiping solution of intaglio printing presses. A known solution which does not involve an ultra-filtration process is also disclosed in European Patent Application No. EP 1 092 684 A2. This other solution involves a flocculating process similar to the one described in European Patent Application No. EP 0 826 502 A1 using a mixture of calcium chloride (CaCl2) and ferric chloride (FeCl3) as flocculating agent. The use of calcium chloride (CaCl2) within the recycling loop of the system has however demonstrated in practice to be detrimental to the proper operation of the associated intaglio printing press(es) as calcium chloride (CaCl2) favours rusting and oxidisation of metallic parts of the printing press(es), which problems one evidently wishes to avoid as much as possible.
According to European Patent Application No. EP 1 092 684 A2, the flocculated solution is filtered to separate it into sludge and a filtrate, which filtrate is then heated to recover distilled water. This distilled water is in effect the only constituent, out of all components contained in the waste wiping solution, that is recycled, according to this solution, to serve as raw material for producing new wiping solution. This means that the recycling yield of this solution is comparatively lower than that of other known solutions where more than simply distilled water is recycled.
European Patent Application No. EP 1 092 684 A2 additionally provides for a further treatment of concentrated solution that is produced as a result of heating of the filtrate, which further treatment involves supplying the concentrated solution onto a heating drum and heating this concentrated solution to evaporate the liquid components of the concentrated solution into vapour, while permitting solid residues (or “salts”) contained in the concentrated solution to be deposited on the surface of the heating drum.
German Patent Application No. DE 101 63 340 A1 discloses a process and installation for recycling cleaning solution used for cleaning printing presses wherein an emulsion-breaker (“Emulsionsspalter” in German) is added to the waste cleaning solution and wherein a phase separation of the waste cleaning solution is carried out using a decanting unit or, by way of an alternative, using a centrifuge. No particular reference is made in this document to the treatment of wiping solution of intaglio printing presses.
There therefore remains a need for further improvements of installations and processes for recycling wiping solution of intaglio printing presses.